1. Field
Embodiments of the disclosure relate generally to the field of reduction of solar absorptivity in surfaces and more particularly to embodiments and fabrication methods for use of riblets or other high-aspect-ratio (height to base) surface microstructures having coloring and spectral tailoring for reduced solar absorptivity extending from a base layer.
2. Background
Strength and durability of composite structures may be impacted by excessive heating due to effects of solar absorptivity of the surface of the structure or coatings applied to the surface due to color or spectral properties. Reducing solar absorptivity of the exterior surface/paint/appliqué is an approach to reduce the solar heating issue. Use of low solar absorptivity coloration of coatings such as “bright white” paint, appliqués or matrix impregnation reduces the impact of solar heating. However, many applications for structural composites are enhanced by various coloration of the surface other than bright white. However, nearly half of the energy in the solar spectrum is in the visible, so the solar absorptivity is very sensitive to visual color and intensity. Dark colors absorb almost all of the energy in the visible band, and they usually absorb a high percentage of the solar energy in the Near Infrared and UV as well, resulting in excessive heating of the surfaces. To accommodate the additional thermal load due to solar heating, additional structural strength may be required which may impact weight and cost. Significant benefit has previously be obtained by spectrally tailoring the optical properties of a paint or coating, i.e. reflecting the UV and/or Near Infrared while allowing only the visible absorption (so that the desired colors are still possible) while reducing the requirement for structural enhancement. This approach has been used to design, scale up and manufacture a paint coating to reduce the thermal load on aircraft structures when the aircraft could not be white. Often greater reductions are needed than can be achieved by tailoring surface solar absorptivity alone.
Increasing fuel efficiency in modern aircraft is being accomplished through improvement in aerodynamic performance and reduction of structural weight. Recent advances in the use of microstructures such as riblets on aerodynamic surfaces have shown significant promise in reducing drag to assist in reducing fuel usage. Riblets have various forms but advantageous embodiments may be ridge-like structures that minimize drag on the surface of an aircraft. Riblets may be used in areas of a surface of an aircraft where turbulent regions may be present. Riblets may limit circulation causing a breakup of large scale vortices in these turbulent regions near the surface in the boundary layer to reduce drag. In certain tested applications riblets have been pyramidal or inverted V shaped ridges spaced on the aerodynamic surface to extend along the surface in the direction of fluid flow. Riblet structures have typically employed polymeric materials, typically thermoplastic or thermoset polymers.
Surface appliqués for lightning protection, EMI shielding, P-static mitigation and replacement of the decorative/protective paint are being employed on aircraft and other vehicles.
It would therefore be desirable for an appliqué with multiple functions including reducing solar heating, aerodynamic drag reduction, lightning protection, EMI shielding, P-static mitigation and paint replacement.